The present invention relates to a developing method comprising cleaning off a developer remaining on a photosensitive member by a developing device at the same time as development in an image forming apparatus such as an electrophotographic apparatus making good use of electrophotography or an electrostatic recording apparatus, and a non-magnetic one-component developer used in this developing method.
An image forming apparatus according to electrophotography is generally equipped with a photosensitive member (electrostatic latent image-bearing member), a means for evenly and uniformly charging the surface of the photosensitive member, an exposing means (electrostatic latent image-forming means) for writing an electrostatic latent image on the charged surface of the photosensitive body, such as a laser device, a developing means for developing the electrostatic latent image on the photosensitive member with a developer (toner), a transferring means for transferring the developer image developed to a transfer medium such as transfer paper or an OHP sheet, and a fixing means for fixing the toner image transferred on the transfer medium. Many of such conventional image forming apparatus are also equipped with a cleaning device for removing the developer remaining on the photosensitive member.
Specifically, for example, such an image forming apparatus equipped with a cleaning device as illustrated in FIG. 2 is generally used. The image forming apparatus has a structure that a cleaning device 202, a charging device 203, an exposing device 204, a developing device 205, a transferring device 206, etc. are arranged around a photosensitive member 201. In order to form an image with the image forming apparatus, the surface of the photosensitive member 201 is first evenly and uniformly charged to a plus (+) or minus (xe2x88x92) charge by the charging device 203. The charging device illustrated in FIG. 2 is a charger according to corona discharge. Image exposure is then conducted by the exposing device 204 to form an electrostatic latent image on the photosensitive member 201. Electric resistance is lowered at an area (exposed region) of the photosensitive member, to which light has been applied by the exposure, and so the plus or minus charge vanishes.
The electrostatic latent image formed on the photosensitive member 201 is developed by the developing device 205. The developing device 205 illustrated in FIG. 2 is constructed by a developing roller 208, a layer-thickness regulating blade 209 for developer, a developer (toner) 210, a developer feed roller 212, a casing 211 for containing these members therein, etc. The layer thickness of the developer transferred to the developing roller 208 by the rotation (rotating direction C) of the feed roller 212 is controlled by the blade 209 coming into contact with the developing roller 208, whereby a thin layer of the developer is formed on the developing roller 208. The developer on the developing roller 208 is applied to the electrostatic latent image by bringing the developing roller 208 (rotating direction B) and the photosensitive member 201 (rotating direction A), which rotate to reverse directions to each other, into contact with each other, thereby forming a developer image on the photosensitive member 201.
In the case of ordinary normal development system, a charge remains only on image portions such as characters corresponding to original due to image exposure. A developer charged to a polarity opposite to the charge of this electrostatic latent image is applied to form the developer image. The developer image on the photosensitive member 201 is electrostatically transferred to a transfer medium 207 by the transferring means 206. The transferring device illustrated in FIG. 2 is a charger according to corona discharge and serves to charge the transfer medium 207 to a polarity opposite to the developer to transfer the developer image from the surface of the photosensitive member to the transfer medium. The developer image transferred to the transfer medium 207 is fixed to the transfer medium by any of various methods such as heating, pressing and use of solvent vapor.
In the transferring step, a part of the developer. remains on the photosensitive member 201 without being transferred to the transfer medium 207. Therefore, this kind of image forming apparatus is generally equipped with a cleaning device 202 for removing the developer remaining on the photosensitive member 201. After the transferring step, the developer remaining on the photosensitive member 201 is removed by a cleaning blade 202a of the cleaning device 202 and accumulated. in the cleaning device 202.
Such an image forming apparatus has involved such problems that (1) the developer accumulated in the cleaning device must be disposed, and so maintenance is complicated, (2) pollution about the image forming apparatus and of environment is incurred by flying of the developer, (3) the photosensitive member is abraded or damaged by its contact with the cleaning blade, and so image quality is deteriorated as the number of printed sheets increases, (4) the developer ground by the contact with the cleaning blade adheres to the photosensitive. member to tend to cause toner filming,.and (5) the degree of freedom of design in the image forming apparatus is lowered by the provision of the cleaning device, thereby interfering with the miniaturization of the image forming apparatus.
In order to solve such problems as described above, there have therefore been proposed developing methods (cleaning systems concurrent with development) that a one-component developer is used to recover the developer remaining on a photosensitive member by a developing device at the same time-as development (Japanese Patent Application Laid-Open Nos. 203182/1987and 7972/1991). When such a cleaning system concurrent with development is adopted, there is no need to provide a cleaning device equipped with a cleaning blade. As described above, the surface of the photosensitive member is evenly and uniformly charged to a plus (+) or minus (xe2x88x92) charge. In the cleaning system concurrent with development, a developing roller carrying the developer charged to the same polarity as the charged polarity of the photosensitive member is arranged in opposition to the photosensitive member, and an exposed region on the photosensitive member, which has been charged, is developed with the developer, and at the same time the remaining developer adhered to a non-exposed region on the photosensitive member is removed by suction on the side of the developing roller to conduct cleaning.
FIG. 1 illustrates an exemplary image forming apparatus for carrying out such a developing system. The details of the image forming apparatus illustrated in FIG. 1 will be described subsequently. Here, the principle of the cleaning system concurrent with development is described with reference to FIG. 1 and FIG. 3. As illustrated in FIG. 3(A), the surface of a photosensitive member 1 is first evenly and uniformly charged to, for example, a plus charge by means of a charging device 3 (for example, charging roller). Image exposure is then conducted by a exposing device 4 to form an electrostatic latent image on the photosensitive member 1. As illustrated in FIG. 3(B), the charge vanishes at an exposed region 302 on the photosensitive member due to photoconductivity, and so the surface potential thereof is lowered. The plus (+) charge remains at a non-exposed region 301. As illustrated in FIG. 3(C), a thin layer 10a of a developer (toner) charged to a plus (+) charge is formed on the surface of a developing roller 8.
The surface potential of the non-exposed region 301 and the surface potential of the exposed region 302 on the photosensitive member 1 are regarded as Vo and Vq, respectively. The development bias voltage applied to the developing roller 8 is regarded as Vb, and the surface potential Ve of the developing roller 8 is regarded as being equal to the bias voltage Vb. The electrostatic latent image on the photosensitive member is subjected to reversal development with the one-component developer (toner) charged to the same polarity as the polarity (i.e., the polarity of the photosensitive member charged) of the charge of the non-exposed region.
In this reversal development system, the intensity of the respective surface potentials is controlled so as to satisfy the relationship of the following inequality (I):
|Vo| greater than |Ve| greater than |Vq|xe2x80x83xe2x80x83(I)
However, Vo, Ve and Vq have the same polarity. In the exposed region 302 on the photosensitive member 1, force according to the potential difference |Vexe2x88x92Vq| toward the direction of the photosensitive member acts on the developer 10a on the developing roller 8, and so the developer adheres to the exposed region 302, whereby development is conducted. Even when a part of the developer remains on the photosensitive member after the transferring step, and the remaining developer 10b adheres to a non-exposed region 301 in the nest exposing step, force according to the potential difference |Voxe2x88x92Ve| toward the direction of the developing roller acts on the remaining developer 10b in a developing step, and the remaining developer 10b is recovered by its adhesion to the developing roller 8. As a result, the remaining developer is also cleaned off at the same time as the reversal development as illustrated in FIG. 3(D). According to this cleaning system concurrent with development, the conventional cleaning devise becomes unnecessary. Even when the charge polarities of the surface of the photosensitive member and the developer are changed to minus (xe2x88x92), the same result can be yielded.
In the developing method according to this cleaning system concurrent with development, a developer layer as thin as possible is formed with an even thickness on the developing roller 8 by a layer-thickness regulating blade 9 for toner. As the developer, a non-magnetic one-component developer comprising a binder resin and a colorant and containing no magnetic powder is preferably used because it has a high electric resistance.
In the cleaning system concurrent with development, however, it is difficult to satisfy both image density and cleaning ability at the same time. When a potential difference |Vexe2x88x92Vq| is made great in order to achieve sufficient image density, a potential difference |Voxe2x88x92Ve| required for recovery of the remaining developer is lowered, and so cleaning becomes insufficient, and ghost images are liable to occur. When a potential difference |Voxe2x88x92Ve| is made great in order to improve the cleaning ability, a potential difference |Vexe2x88x92Vq| required for development is lowered, and so satisfactory image density cannot be achieved. When the transferability of a developer image on the photosensitive member to a transfer medium is insufficient, and an amount of the remaining developer increases, it is necessary to control the respective surface potentials Vo, Ve and Vq and moreover control the layer thickness of a layer formed on the developing roller and a rotational ratio of the photosensitive roller to the developing roller for the purpose of satisfying both image density and cleaning ability to properly maintain the development amount of the developer, thereby lessening the amount of the remaining developer after transfer. However, the scopes of control conditions of the respective surface potentials, layer thickness of the developer and rotational ratio are limited, and so it has been difficult to properly control these conditions. In addition, the remaining developer is used repeatedly without discarding it in the cleaning system concurrent with development, and so the properties of the developer tend to be deteriorated as the number of printed sheets increases in continuous printing or repeated printing over a long period of time, and it has been difficult to maintain high image quality.
In order to solve such a problem, it has heretofore been proposed to use a non-magnetic one-component developer containing a spherical polymerized toner obtained by a suspension polymerization process (Japanese Patent Application Laid-Open No. 188637/1993). The use of this non-magnetic one-component developer has involved problem that the flowability of the developer itself is deteriorated as the number of printed sheets increases, and the resulting image tends to cause increase in fog and blur though satisfactory image properties can be achieved in an initial stage according to the cleaning system concurrent with development.
In order to improve continuous printing ability and the like, various improvements have been proposed as to toners according to the grinding process. The ground toners are developers produced by melting and mixing various components such as a colorant, a charge control agent and a parting agent in a thermoplastic resin into a composition, grinding the composition and classifying the ground product. It has been proposed to control classification conditions to provide a ground toner in which the volume average particle diameter is 7 to 12 xcexcm, the proportion of particles having a particle diameter of at most 6 xcexcm is at most 13% by number, the proportion of particles having a particle diameter of at least 16 xcexcm is at most 2% by volume, and the standard deviation of particle diameter volume distribution is at most 2.7 (Japanese Patent Application Laid-Open No. 22138/1996). It is reported in this publication that when a non-magnetic one-component developer comprising the ground toner is used, image deterioration (increase of roughness and fog in images, etc.) is hard to occur even in continuous printing or long-term repeated printing.
Similarly, in Japanese Patent Application Laid-Open Nos. 3244/1966, 129437/1983, 877/1990 and 22138/1996, it is also shown that a ground toner of a one-component or two-component system, in which the particle diameter distribution and the content of fine or coarse particles have been controlled, is useful in continuous printing and long-term repeated printing.
However, the mere control of the particle diameter distribution and the content of fine or coarse particles in the ground toner has been insufficient for the maintenance of high image quality in continuous printing and long-term repeated printing. In addition, in the ground toner, the form of particles is indefinite, and a ratio (average value) of length to breadth is generally greater than 1.3. The transfer efficiency of such a ground toner is as low as 60 to 80%. Therefore, the use of such a ground toner as a developer for the cleaning system concurrent with development has involved a problem that the scopes of conditions for properly controlling the respective surface potentials Vo, Ve and Vq and moreover properly controlling the layer thickness of the developer formed on the developing roller and rotational ratio of the photosensitive member and the developing roller are limited.
The phenomenon that the flowability of a developer is lowered, or fog and blur increase in images is attributable to the cleaning system concurrent with development. In an image forming apparatus making use of this developing method, a developer remaining on a photosensitive member after a transferring step is recovered by a developing device because no conventional cleaning device is provided, and the developer is used repeatedly. As a result, contents of abnormal developers such as a developer deteriorated in flowability, a developer low in charging properties and a developer charged to an opposite polarity increases as the number of printed sheets increases. When development is conducted with such developers, fog and blur increase in the resulting image.
In order to prevent deterioration of image quality by long-term use, there has heretofore been proposed a non-magnetic one-component developer comprising a polymerized toner in which the average particle diameter is 1 to 12 xcexcm, a proportion of particles having a particle diameter of at least 16 xcexcm is at most 10% by weight, and a coefficient of variation defined as a proportion of a standard deviation of the toner to the average particle diameter is at most 20% (Japanese Patent No. 2751210). In order to provide a polymerized toner having such a sharp particle diameter distribution, however, a classifying step is generally required, and a yield is greatly lowered because coarse particles are removed. It is desirable from the viewpoint of yield of the polymerized toner that the coefficient of variation be higher than 20%, preferably about 23 to 35%. There is a demand for development of a polymerized toner that can be provided without lowering the yield of the polymerized toner and provide high-quality images when it is applied to the cleaning system concurrent with development.
It is an object of the present invention to provide a non-magnetic one-component developer suitable for use in the cleaning system concurrent with development.
More specifically, the object of the present invention is to provide a non-magnetic one-component developer which does not cause lowering of flowability and image quality even in continuous printing or long-term repeated printing when it is applied to a developing method according to the cleaning system concurrent with development.
Another object of the present invention is to provide a developing method which is a developing method according to a cleaning system concurrent with development and does not cause fog and blur in the resulting image by continuous printing or long-term repeated printing.
The present inventors have carried out an extensive investigation with a view toward overcoming the above-described problems involved in the prior art. As a result, the present inventors have conceived a non-magnetic one-component developer comprising a substantially spherical polymerized toner the average particle diameter, particle diameter distribution, form, etc. of which have been controlled within respective specific selected ranges. The non-magnetic one-component developer according to the present invention is suitable for use as a developer for the cleaning system concurrent with development and does not cause lowering of flowability and image quality even in continuous printing or long-term repeated printing. The present invention has been led to completion on the basis of these findings.
According to the present invention, there is thus provided a non-magnetic one-component developer comprising a substantially spherical polymerized toner which contains at least a binder resin and a colorant and has the following features:
(a) the volume average particle diameter being 5 to 10 xcexcm
(b) the proportion of particles having a particle diameter of at most 5 xcexcm being at most 25% by number;
(c) the proportion of particles having a particle diameter of at least 16 xcexcm being at most 2% by volume;
(d) the standard deviation of the number particle diameter distribution being at most 1.8; and
(e) the spheroidicity represented by a ratio (dl/ds) of a length (dl) to a breadth (ds) in a particle being 1.0 to 1.3.
The non-magnetic one-component developer according to the present invention is suitable for use as a developer for the cleaning system concurrent with development. The polymerized toner is preferably that obtained by subjecting a monomer composition containing at least a polymerizable monomer and a colorant to suspension polymerization. The polymerized toner particularly. preferably has a core-shell structure. It is preferable that a charge control resin containing a polar group and soluble in the polymerizable monomer be used as a charge control agent.
According to the present invention, there is also provided a developing method comprising arranging a developing roller carrying a developer charged to the same polarity as the charged polarity of the charged surface of a photosensitive member in contact with the photosensitive member, exposing the photosensitive member, and then developing an exposed region on the photosensitive member with the developer, and at the same time removing a remaining developer adhered to a non-exposed region on the photosensitive member by suction on the side of the developing roller to conduct cleaning, wherein the developer is a non-magnetic one-component developer comprising a substantially spherical polymerized toner which contains at least a binder resin and a colorant and has the following features:
(a) the volume average particle diameter being 5 to 10 xcexcm;
(b) the proportion of particles having a particle diameter of at most 5 xcexcm being at most 25% by number;
(c) the proportion of particles having a particle diameter of at least 16 xcexcm being at most 2% by volume;
(d) the standard deviation of the number particle diameter distribution being at most 1.8; and
(e) the spheroidicity represented by a ratio (dl/ds) of a length (dl) to a breadth (ds) in a particle being 1.0 to 1.3.
It is preferable that the photosensitive member and the developing roller be rotated in the same direction as each other at the contact portion thereof, and the rotational ratio between both can be controlled.